The present invention relates generally to planar lightwave circuits. More particularly, the present invention relates to a method and system for a reduced polarization dependent wavelength shift/polarization dependent loss of planar lightwave circuit.
Planar lightwave circuits comprise fundamental building blocks for the newly emerging, modern fiberoptic communications infrastructure. Planar lightwave circuits are innovative devices configured to transmit light in a manner analogous to the transmission of electrical currents in printed circuit boards and integrated circuit devices. Examples include arrayed waveguide grating devices, integrated wavelength multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and the like.
Planar lightwave circuits generally involve the provisioning of a series of embedded optical waveguides upon a semiconductor substrate (e.g.,silicon), with the optical waveguides fabricated from one or more silica glass substrate layers, formed on an underlying semiconductor substrate. Fabrication techniques required for manufacturing planar lightwave circuits using silica glass substrates is a newly emerging field. Electronic integrated circuit type (e.g., CMOS) semiconductor manufacturing techniques have been extensively developed to aggressively address the increasing need for integration in, for example, the computer industry. This technology base is currently being used to make planar light circuits (PLCs). By using manufacturing techniques closely related to those employed for silicon integrtated circuits, a variety of optical circuit elements can be placed and interconnected on the surface of a silicon wafer or similar substrate. This technology has only recently emerged and is advancing rapidly with leverage from more mature tools of the simiconductor-processing industry.
A PLC optical waveguide comprises a silica glass substrate, at least one core waveguide formed thereon, and a cladding layer covering the core waveguide, wherein a certain amount of at least one dopant is added to both the core waveguide and the cladding layer so that the refractive index of the core waveguide is higher than that of the cladding layer. Fabrication of conventional optical waveguides involves the formation of a silica glass substrate, usually upon a silicon semiconductor wafer. A doped SiO2 glass layer is deposited and is fabricated (e.g., with lithography processes) into a waveguide core, wherein a mask is formed on the doped SiO2 glass layer. The waveguide core is patterned on the substrate typically by reactive-ion etching to remove the excess doped SiO2 glass. The substrate is subsequently heated to an anneal temperature to stabilize the refractive index of the waveguide core. A SiO2 cladding layer is then formed. Finally, the wafer is cut into multiple planar lightwave circuit dies and packaged according to their particular applications.
Prior art FIG. 1 shows a cross-section view of a conventional planar optical waveguide fabricated using a silica glass substrate. As depicted in FIG. 1, the planar optical waveguide includes a doped SiO2 glass core 10 formed over a SiO2 silica glass substrate 12. A SiO2 cladding layer 11 covers both the core 10 and the substrate 12. As described above, the refractive index of the core 10 is higher than that of the cladding layer 11 and the substrate 12. Consequently, optical signals are confined axially within core 10 and propagate lengthwise through core 10.
A well-known problem with many planar lightwave circuits is the polarization sensitivity of the device. For example, with arrayed waveguide grating (AWG) devices, integrated wavelength multiplexers/demultiplexers, and the like, due to the fact that an optical signal propagating through an optical fiber has an indeterminate polarization state, the switching/routing devices must be substantially polarization insensitive. However, due to stress imposed upon a silica substrates (e.g., from the fabrication process) and other factors, planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) propagation modes. For AWG devices, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be 3 nm or larger. As wavelength division multiplexing progress towards smaller and smaller channel spacing dimensions (from 1.6 nm to 0.8 nm or even less), even a small polarization dependent wavelength shift (e.g. 0.3-0.4 nm) is potentially troublesome.
Polarization dependent wavelength shifts causes mismatches in the propagation constants for TE and TM modes. This mismatch causes a polarization dependent loss, wherein either the TE or TM mode is attenuated within the optical waveguide structures to a greater degree than the other. The wavelength shift between TE and TM and the different losses experienced causes significant signal degradation. Thus, the polarization dependent wavelength shift and the polarization dependent loss characteristics of a planar lightwave circuit, particularly a dense WDM device (e.g., an AWG multiplexer or demultiplexer), is an important measure of the device""s performance.
Prior art FIG. 2 shows a graph depicting polarization dependent wavelength shift and polarization dependent loss for TE and TM propagation modes. As depicted in FIG. 2, a TE signal component and a TM signal component are graphed after having experienced phase dependent wavelength shift (PDW) and phase dependent loss (PDL), from, for example, propagation along the core 10 of the planar optical waveguide of FIG. 1. The vertical axis of the graph shows insertion loss in decibels and the horizontal axis shows wavelength. As described above, the difference in propagation constants for the TE and TM signal components results in a PDW wavelength shift 21 in the spectral response peak between the TE and TM signal components. This wavelength shift in turn causes a PDL loss 22.
Thus what is required is a solution that matches the TE and TM propagation modes of an optical signal within a planar lightwave circuit. What is required is a solution that minimizes polarization dependent wavelength shift within a planar lightwave circuit. What is further required is a solution that minimizes polarization dependent loss within a planar lightwave circuit. The required solution should significantly increase the performance of polarization sensitive optical waveguide devices. The present invention provides a novel solution to the above requirements.
The present invention provides a solution that matches the TE and TM propagation modes of an optical signal within a planar lightwave circuit. The present invention provides a solution that minimizes polarization dependent wavelength shift within a planar lightwave circuit. Additionally, the solution of the present invention minimizes polarization dependent loss within a planar lightwave circuit and significantly increases the performance of high-precision optical devices such as AWG demultiplexers/multiplexers.
In one embodiment, the present invention comprises an optical core layer over-etch process for making a polarization insensitive optical waveguide structure. An optical core layer is formed on a substrate, wherein the optical core layer has a higher refractive index than the substrate. A mask is then formed over the optical core layer. The unmasked areas of the optical core layer are then over-etched to define the core, wherein the over-etching removes the unmasked area of the optical core layer and a portion of the substrate disposed beneath the unmasked area, along with defining the optical core. The mask is subsequently removed from the optical core. A cladding layer is then formed over the optical core and the substrate, the cladding layer having a lower refractive index than the optical core. The cladding layer, the optical core, and the substrate fabricated in this manner together form a polarization insensitive optical waveguide structure.
The amount of over-etching can be controlled in order to control an amount of substrate disposed beneath the unmasked area of the optical core layer that is removed. The amount of substrate removed, in turn, controls the polarization sensitivity of the optical waveguide structure. The amount of the portion of the substrate removed during the over-etching can be determined to minimize the polarization dependent wavelength shift and the polarization dependent loss of the optical waveguide structure (e.g., via computer simulations, empirical tests, etc.). The amount of the portion of the substrate removed during the over-etching can be determined in accordance with a blanket stress of the cladding layer. The over-etching can be within a range between 7.5 percent and 30 percent depending on manufacturing process of planar lightwave circuits such as AWG.
In this manner, the controlled percentage of over-etch matches the TE and TM propagation modes of an optical signal within the optical waveguide structure. The matching of the TE and TM signal components minimizes polarization dependent wavelength shift polarization dependent loss within a planar lightwave circuit.